This invention concerns a method for the expansion of the range of the depth of focus in the optical imaging of three-dimensional objects, as well as an apparatus for its implementation.
A method for increasing the range of the depth of focus is known from German Pat. No. 2,301,800, where, in a first step, an object to be imaged is uniformly moved along the optical axis of the system, whereby, through the superposition of various defocussed images, a modified image of the object is produced. In a second step, the image thus obtained is subjected to a high pass filter, resulting in a compensation in the contrast decrease which has developed at high local frequencies. The integration carried in the first step of this known method can be performed on a photoplate or, if the motion of the object is fast enough, on a vidicon. If the range of integration becomes greater than the depth of the object, the integrated image will be characterized in that all level positions of the object are imaged with almost equal contrast transfer function, which decreases approximately as the function 1/s at higher local frequencies. For this reason, the integrated image, which evidences low contrast at high local frequencies, can be filtered back by a second step in accordance with the known method. In this case, it is necessary that the characteristic of the filter which is used has an essentially linearly increasing response with a maximum amplification V at V-times the expansion of the depth of focus.
In the method disclosed in German Pat. No. 2,301,800, because of unavoidable noise which occurs, for example, in the electronic high pass filtering of a vidicon signal, the range of the expandable depth of focus is limited to a range of ten to fifteen times. Furthermore, the motion of the object must be exactly in the direction of the optical axis of the imaging system, in rapid sequence, which necessitates using expensive apparatus.